This invention concerns a process for preparing a polyhydroxycarboxylic acid and, more specifically, it relates to a process for preparing a polyhydroxycarboxylic acid comprising a step of heating to crystallize pellets of a polyhydroxycarboxylic acid under a specified condition and a step of solid phase polycondensation thereof under a specified condition.
1. Technical Background
In recent years, disposal of wastes has provided problems in relation with environmental preservation. Particularly, molding products and processed products of generally used polymeric materials bring about problems in that they are not biologically degraded or disintegrated when filled as wastes but remain substantially permanently as obstacles and, further, additives such as plasticizers are leached to contaminate environments.
Further, it has been closed up more and more in recent years that when they are burnt as the wastes, high heat calorie generated from combustion damages furnaces and discharged smokes and gases generated by combustion may possibly cause atmospheric pollution, destruction of ozone layers, global warming and acid rain.
In view of the situations described above, a demand has been increased for such polymeric materials that are decomposable when filled as wastes after the use although they are tough and strong, or give less combustion heat when they have to be burnt and do not damage the furnaces. However, it can not be said that polymeric materials capable of satisfying such a demand are easily available at present.
By the way, polyhydroxycarboxylic acids can be mentioned, for example, as those capable of satisfying the demand described above. Among them, polylactic acid as a kind of the polyhydroxycarboxylic acids has high transparency, high strength and has an easily hydrolyzable characteristic under the presence of water and, when it is used as the general-purpose resin, it is decomposed without contaminating the environment after disposal, thus gentle to the environment. Further, a case of being dwelt in a living body as a medical material, it can be decomposed and absorbed in the living body giving no toxicity to the living body after attaining the purpose, so that such excellent natures gentle to the living body has already been noted prior to the filing of this application. For example, Japanese Open-Laid Patent Application No. Hei 5-255488 discloses a preparation method of using a so-called solid phase polymerization, of conducting polycondensing reaction for polyhydroxycarboxylic acids (so-called low molecular weight polymer) below the melting point thereof.
2. Problems in the Prior Art
By the way, in the prior art, it is prepared by way of a solid phase polycondensation reaction using a polyhydroxycarboxylic acid of low molecular weight obtained by a liquid phase polycondensation step. However, since the polyhydroxycarboxylic acid obtained by the liquid phase polycondensation reaction has a low molecular weight, it is less crystallizable and may cause blocking or fusion when used as it is to the solid phase polycondensation reaction step. In view of the above, a crystallizing step is disposed before the solid phase polycondensation reaction step and, after heating to crystallize the polyhydroxycarboxylic acid by way of the liquid phase polycondensation reaction step, it is used for the solid phase polycondensation reaction step.
However, for rapid conduction of the crystallizing step, when the temperature is elevated rapidly or heating is effected at high temperature, since blocking or fusion of the polyhydroxycarboxylic acid pellets takes place in the system, a relatively moderate temperature elevation condition has to be adopted. As a result, the productivity can not be improved as intended, so that a preparation method of more economical performance and excellent in productivity has been demanded.
Further, since the polyhydroxycarboxylic acid used for the solid phase polycondensation reaction has low molecular weight and has low melting viscosity, particles of uniform shape can not be obtained by a strand cut system, underwater cut system or hot cut system used in the pelleting of usual polymers.
The polyhydroxycarboxylic acid of low molecular weight can not be formed into pellets but has to be handled in the shape of a fine powder such as powders or flakes. That is, the powdery form is difficult to be handled with, preparation of a polyhydroxycarboxylic acid of high molecular weight by the solid phase polycondensation reaction in an economical preparation process is difficult, as well as the processing stability was poor since the fine powder can not be supplied stably to molding machines.
Accordingly, an object of the invention is to overcome the foregoing problems in the prior art and provides a process for preparing a polyhydroxycarboxylic acid of high molecular weight efficiently.
The present inventors have made an earnest study for solving the foregoing subject and, as a result, have found conditions capable of efficiently preparing a polyhydroxycarboxylic acid of high molecular weight for the process for preparing a polyhydroxycarboxylic acid of high molecular weight, with no blocking or fusion of pellets of the polyhydroxycarboxylic acid of low molecular weight in the system, by way of a step of heating to crystallize pellets of a polyhydroxycarboxylic acid of low molecular weight under a specified condition and, successively, a step of conducting solid phase polycondensation reaction under a specified condition.
Further, it has been found that the crystallizing step and the solid phase polycondensation reaction step can be conducted efficiently when the pellet of a polyhydroxycarboxylic acid of low molecular weight used in the crystallizing step and the solid phase polycondensation reaction step has a special form or specified layer structure obtained under the specified preparation conditions.
As the result, the invention has been accomplished.
That is, this invention provides the following features (1) to (7).
(1) A process for preparing a polyhydroxycarboxylic acid of high molecular weight having a weight average molecular weight of 50,000 to 1,000,000, comprising a crystallizing step of heating to crystallize a polyhydroxycarboxylic acid of low molecular weight obtained by a liquid phase polycondensation reaction step and a solid phase polycondensation reaction step of heating to a temperature not lower than the crystallizing temperature in which the crystallizing step comprises:
(A) a step of heating pellets of a polyhydroxycarboxylic acid having a weight average molecular weight of 2,000 to 100,0000 under stirring at a temperature not lower than the glass transition point and not higher than the melting point, and heating the same till an exothermic peak area corresponding to the heat of crystallization of at most 10 J/g or less and an endothermic peak area corresponding to the heat of melting of 38 J/g or more as recognized upon conducting temperature elevation analysis of the polyhydroxycarboxylic acid by using a differential scanning calorimeter and, successively,
(B) a step of heating to elevate the temperature of the polyhydroxycarboxylic acid to a temperature for the solid phase polycondensation reaction and
the solid phase polycondensation reaction step comprises:
a step of conducting solid phase polycondensation of the polyhydroxycarboxylic acid at the temperature not higher than the start temperature of the endothermic peak as recognized upon conducting temperature elevation analysis of the polyhydroxycarboxylic acid by using a differential scanning calorimeter.
(2) A process for preparing a polyhydroxycarboxylic acid having a weight average molecular weight of 50,000 to 1000,000 as defined in (1) above, wherein the average particle size of the pellet of the polyhydroxycarboxylic acid before crystallization is within a range from 0.1 mm to 20 mm and the standard deviation of the particle diameter is within 1 mm.
(3) A process for preparing a polyhydroxycarboxylic acid having a weight average molecular weight of 50,000 to 1000,000 as defined in (1) or (2), wherein the pellets of the polyhydroxycarboxylic acid before crystallization are pellets of particulate polyhydroxycarboxylic acid prepared by melting a polyhydroxycarboxylic acid having a weight average molecular weight of 2,000 to 100,000, and dropping and cooling to solidify the molten polyhydroxycarboxylic acid having a melting viscosity of 0.01 to 100 poise.
(4) A process for a preparing polyhydroxycarboxylic acid having a weight average molecular weight of 50,000 to 1000,000 as defined in (3) above wherein the melting temperature is from 140xc2x0 C. to 200xc2x0 C.
(5) A process for preparing a polyhydroxycarboxylic acid having a weight average molecular weight of 50,000 to 1000,000 as defined in (1) above, wherein the pellet of the polyhydroxycarboxylic acid before crystallization is a pellet comprising (1) a inner core portion and (2) an outer coating portion covering the inner core portion, in which
(I) the inner core portion comprises a resin containing a polyhydroxycarboxylic acid having a weight average molecular weight (Mw) within a range from 2,000 to 30,000 and
(II) the outer coating portion comprises a resin containing a polyhydroxycarboxylic acid having a weight average molecular weight (Mw) of 30,000 to 100,000, in which
the resin constituting the inner core portion has a melting point lower than that of the resin constituting the outer coating portion or shows no melting point.
(6) A process for preparing a polyhydroxycarboxylic acid having a weight average molecular weight of 50,000 to 1000,000 as defined in any one of (1) to (5) above, wherein the hydroxy-carboxylic acid is an aliphatic hydroxy-carboxylic acid.
(7) A process for preparing a polyhydroxycarboxylic acid having a weight average molecular weight of 50,000 to 1000,000 as defined in (6) above, wherein the aliphatic hydroxy-carboxylic acid contains lactic acid.